I. Field of the Invention
This invention relates to a method of and apparatus for the precipitation and classification of solids from mother liquors (slurries) in industrial processes. More particularly, although not exclusively, the invention relates to the precipitation and classification of alumina hydrate from aluminate liquors during the Bayer process for the production of alumina from bauxite.
II. Background Art
In the Bayer process, hydrated aluminum oxide in bauxite is digested at elevated temperature and pressure with concentrated sodium hydroxide solution to form a solution of sodium aluminate. The solution is separated from the remaining insoluble materials (red mud) and is then cooled so that it becomes super saturated. Seed crystals of alumina hydrate are added and a precipitate is produced. Coarse crystals of the resulting precipitate are separated as a product stream intended for drying and calcination to produce alumina. Fine crystals are removed for use in the process and the remaining sodium aluminate (spent liquor) is recycled to the digestion step for use with the caustic hydroxide solution.
In the American variant of the classic Bayer process, it is normal to classify the precipitation discharge stream into large particles (product), a coarse seed fraction and a fine seed fraction. The fine seed is then subjected to an agglomeration process in high ratio sodium aluminate liquor to reduce the number of particles present. This allows the production of a coarse strong product which is not otherwise attainable.
Commercial pressure to increase liquor (sodium aluminate) productivity, and hence the production yield from existing equipment, leads to the use of higher and higher concentrations of caustic sodium hydroxide. The desire to complete the crystallization in a defined time period then leads to higher and higher seed charges. The resultant higher solids content in the precipitation discharge stream makes the conventional separation and classification equipment difficult to operate and/or inefficient or ineffective. In particular, the separation of a fine fraction from a thick (dense) slurry is difficult, although the separation of large (product) particles from a coarse seed fraction is somewhat less difficult. Difficulties of separation are generally encountered at solids concentrations of greater than about 300 gpl solids.
One way of dealing with the need for a high content of solids in contact with precipitating liquor is to make use of the "dense bed" concept. Basically, this concept is the use of a vessel to produce a higher solids concentration in the vessel than the feed to the vessel by removing an underflow of dense slurry and an overflow containing little solid. Generally, this involves introducing the precipitating liquor and seed charge into a tank or other vessel having a lower zone where the precipitated solids is kept in turbulent agitation suitable for crystal growth and an upper non-turbulent zone where solids from the lower zone may enter and sediment back into the lower zone. By transferring an underflow stream and an overflow stream separately into the next tank in a series, it is possible to increase the solids content in the lower zone in comparison with the solids content of the combined feed streams.
However, it is then still necessary to classify the resulting product into the required product, coarse and fine fractions, and this is difficult at high solids contents, as indicated above.
The use of dense bed precipitators in the Bayer process has been suggested in the past, e.g. in U.S. Pat. No. 3,649,184 issued to Richard H. Featherston on Mar. 14, 1972 and U.S. Pat. No. 4,049,773 issued to Gl.o slashed.r Mejdell et al. on Sep. 20, 1977. These patents are discussed in the following.
U.S. Pat. No. 3,649,184 discloses a process and apparatus for the production of alumina hydrate by creating a different kind of precipitation circuit which uses only one kind of seed material. The process involves continuously introducing aluminate liquor and fine seed slurry into the first of a series of precipitation stages, each stage containing aluminate liquor lower in ratio of alumina to caustic soda concentration than the preceding stage but higher in ratio than the succeeding stage, maintaining in each stage a quiescent region for the separation of fine alumina hydrate particles, and continuously transferring aluminate liquor and entrained fine particles from the quiescent region of each stage to the succeeding stage. The stages employ dense bed precipitators having an upper quiescent region and a lower agitated region. Coarser particle material accumulates in the vessels and each vessel is underflowed periodically to a classification device. Liquid and slightly finer material is returned to the vessel from which it came to maintain the flow conditions in the precipitator chain. This process requires the modification of the entire conventional system to operate as a series of dense bed precipitators.
U.S. Pat. No. 4,049,773 discloses a process in which aluminate mother liquor is passed countercurrent to the direction of feed of aluminum hydroxide crystals through a plurality of stages each with a dense bed type of configuration. Again, this requires a modification of the entire conventional process to operate with dense bed precipitators. Moreover, it is believed that, if attempted on a commercial scale, this invention would cause uncontrollable scaling, coupled with an inability to control the production of the fines necessary to maintain the process.
Accordingly, while the use of dense bed precipitators has been suggested in the past, the systems disclosed have required complete plant refits or the construction of new plants. The capital outlay is thus considerable. Moreover, the results have not been of great benefit. There is therefore a need for a method and apparatus that can handle higher precipitate densities as part of otherwise conventional Bayer plant equipment.